The prior convective heat transfer flue of a boiler is just composed of a flue wall and convective heating surface groups arranged in the flue wall. Flue gas in a furnace enters the convective heat transfer flue via a flue gas inlet of the convective heat transfer flue and advances to a flue gas outlet of the convective heat transfer flue. The travelling path of the flue gas is a straight cylindrical path with a fixed section, with the advancing direction of the flue gas, the flue gas velocity and the zones of the convective heating surface groups swept by the flue gas non-adjustable. As the advancing direction of the flue gas in the convective heat transfer flue is non-adjustable, it leads to a defect that, when the flue gas transversely sweeps over flue gas-water heat transfer tubes arranged in the convective heating surface groups, a vortex (negative pressure) region generated on the backward surfaces of the flue gas-water heat transfer tubes swept by the flue gas is always kept unchanged in position to thus form ash deposition. As the velocity of the flue gas entering the convective heat transfer flue is non-adjustable, it leads to a defect that, at a rated flue gas velocity, the flue gas temperature is greatly reduced after the flue gas interacts with a leading portion of the convective heating surface group in the flue wall, and then the flue gas temperature becomes too low when the flue gas reaches a trailing portion of the heating surface group, so that dew can be formed easily. As the heating surface area of the convective heating surface groups swept by flue gas is non-adjustable, it leads to a defect that, the flue gas is at a relatively low temperature in the starting-up phase or a low-load operation process of the boiler and however needs to sweep all the convective heating surface groups, thereby resulting in a continuous significant reduction of the flue gas temperature, and then when the flue gas reaches the tail heating surface in the flue wall, the flue gas temperature is the lowest to form dew on the tail heating surface.